While the present invention has general application to electromagnetic detector devices for detecting or monitoring metallic material, it has particular utility in devices for detecting metallic debris trapped in the filter element of a filter cartridge to measure wear and predict failure of engines and machinery using filters in their lubricating or cooling systems.
Devices for detecting metallic materials incorporating a linear voltage differential transformer operate on the principal that a normally balanced alternating magnetic field linking opposed coils of the detector circuit will be distorted by metallic materials brought into the proximity of the coils; the distortion produces an imbalance in the signals induced in the opposed coils resulting in an input signal representing the presence of such materials. An alternating magnetic field linking coils of the detector circuit may also be distorted, however, by metallic materials in the proximity producing an imbalance in induced signals not indicative of a specific target material, and also may be distorted by stray magnetic fields which are present in industrial environments. Such distortions of the alternating magnetic field linking the coils of the detector unrelated to a target material or condition being monitored interfere with the proper functioning of the device as a detecting or measuring instrument.